Diaphragms, types, reference electrodes

Figure 3. Divided thermostatted beaker type cell (a) mercury pool cathode with current feeder (platinum wire sealed in glass rod), (b) diaphragm, (c) platinum foil anode, (d) Luggin capillary to reference electrode...

Various permeable membranes were apphed to decrease the rate of electrolyte diffusion under steady state [67], in particular a collodion membrane first proposed in ref. [68] became rather popular. In contrast to ion-selective membranes, no Donnan potential is established when collodion membrane is used. These membranes are introduced to stabilize the system for long-term operation, not to affect UP as is. Many inert viscous substances and porous inert ceramics can serve for the same purpose, and this approach is widely used in modem technologies of miniaturized reference electrodes for applications. Various types of diaphragms (including polymer and ceramics) are used in industrial pH measurements as well [69]. 

At a free-diffusion junction, the LJP reaches a steady value within a few seconds after its formation, is reproducible within 1 mV, and is very stable, although the thickness of the junction expands with time by the mumal diffusions of both the solvents and the electrolytes, as far as the electrolytes on the two sides are the same the drift is within 1 mV h even when the LJP is near to 200 mV [218]. Even when the electrolytes on the two sides are of different kinds, if the concentration on one side is more than 20 times that on the other and if the junction is a free-diffusion type, the LJP is almost decided by the more concentrated electrolyte and it is stable with time [218, 219]. This justifies, to some extent, the use of aqueous reference electrodes for the measurements in nonaqueous solutions. In reality, however, the junctions between different solvents are usually not free-diffusion type but restrained with a diaphragm. The situation is thus complicated the composition of solvents and electrolytes in the diaphragm is indefinite and sometimes a clog of electrolyte is formed, making the LJP less reproducible and less stable. This applies to the case when an aqueous reference electrode is inserted into a nonaqueous solution. 

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